1. Field of the Invention
The present invention pertains to methods and apparatus for detecting the peak of an electrical pulse and more specifically to such methods and apparatus which associate the peak value of the pulse with a value related to the time at which the peak occurs.
2. Setting of the Invention
There are a number of different circumstances in which it is desirable to determine the peak value of an electrical pulse. One technique for determining the peak value is to apply the signal to a peak and hold circuit. In the most simple form, a peak and hold circuit comprises a capacitor having one end grounded with the cathode of a diode connected to the other end. After the pulse is applied to the diode anode, the charge on the capacitor, which is substantially equal to the peak value of the pulse, can be measured. Sometimes it is desirable to associate the peak value of a pulse with a number indicative of the elapsed time between a preceding event and the occurrence of the peak. Such association is desirable when processing electrical pulses generated by a wellbore scanning device sometimes known as a borehole televiewer.
Generally speaking, a borehole televiewer is a downhole tool which includes a transducer mounted on a rotating shaft which generates acoustic pulses responsive to electrical firing pulses applied to the transducer. As the tool is moved axially along the wellbore, downhole circuitry generates periodic firing pulses, thus causing the tool to radially transmit acoustic pulses which helically can the borehole wall. Usually, the acoustic pulse emitted from the transducer strikes the borehole wall and reflects back to the transducer, thus generating a reflected pulse which is detected by the transducer and converted into an electrical pulse. Information regarding the relative times at which the firing pulses and the electrical pulses occur and the magnitude of the electrical pulses is transmitted to the surface on a cable. This information can be used to generate a video display of the borehole wall which the tool has scanned. In one such video display, the time between each firing pulse, which initiates the acoustic pulse, and the detection of the peak of the following electrical pulse is calculated. Each time is assigned a different digital number with the numbers being used to modulate the brightness or z-axis of a video monitor while a horizontal or x-axis sweep is initiated for each full revolution of the transducer. Thus, a new sweep of the video monitor is initiated for each 360.degree. scan of the borehole wall. U.S. Pat. No. 4,463,378 to Rambow and U.S. Pat. No. 3,668,619 to Dennis each disclose borehole televiewer systems which generate signals as described above.
In order to generate such a video display, it is necessary to calculate the time between each firing pulse and the occurrence of the peak of the electrical pulse generated by the reflection of the acoustic pulse.
In Rambow, a scheme for detecting the peak of each electrical pule and calculating the elapsed time between the peak and the preceding firing pulse is disclosed. The Rambow scheme uses a peak and hold circuit for detecting the peak of each electrical pulse. In order to prevent the detection of noise or of a boot signal, which is caused when the electrical pulse amplifier is turned on, an arbitrary threshold level is set below which no peak detection occurs. The occurrence of each firing pulse starts a counter which counts upwardly until the electrical pulse rises above the threshold level at which point the counter is stopped. It should be noted that in the Rambow scheme, the time value associated with the peak value does not occur at the peak, but rather occurs when the signal rises above the threshold. It can thus be seen that when the amplitude of the detected pulses varies, the time value associated with each pulse will also vary, even though there may be the same elapsed time between each pulse peak and the preceding firing pulse. This effect becomes worse as the rise time of the pulse leading edge increases.
Another disadvantage with the Rambow scheme is that low amplitude electrical pulses are sometimes missed because the threshold must be greater than the amplitude of the boot signal generated when the amplifier is turned on. Although higher than the amplitude of the boot signal, the threshold may not be high enough to avoid noise-generated signals and thus the counter may be stopped prematurely as a result of noise having an amplitude greater than the threshold level. This is especially true for noise generated while the signal is falling from the boot signal or firing pulse. Still another disadvantage in Rambow occurs when the electrical pulse has a substantially flat top. In such a case it would be desirable to measure elapsed time to approximately half way between the beginning and end of the top rather than only to the beginning of the pulse top.
Another prior art circuit for detecting the peak of an electrical pulse and associating the detected value with an elapsed time between a prior event and the electrical pulse peak includes a pair of counters. The output of the first counter is provided to a digital-to-analog converter, the output of which is applied to one input of a comparator. The electrical pulse is applied to the other comparator input with the comparator output being fed back to the input of the first counter. The second counter initiates timing at the clock rate upo the occurrence of an initiating pulse. The first counter counts only when the electrical pulse exceeds the output of the digital-to-analog converter, thus generating a count on the output of the first counter proportional to the level of the electrical pulse. Each count of the first counter causes the elapsed time in the second counter to be stored and thus, upon the occurrence of the peak of the electrical pulse, the first counter output is a digital number related to the peak magnitude of the electrical pulse and the second counter output is a digital number related to the elapsed time between the occurrence of the initiating pulse and the occurrence of the peak of the electrical pulse.
The two-counter circuit suffers from a drawback in that for fast raising signals, in the order of 10 microseconds, the first counter must be able to count to approximately its maximum level within the rise time of the signal. If 8-bit resolution is desired, 256 counts must be made within 10 microseconds which, although possible, requires sophisticated (and expensive) state-of-the-art counters. For signals with rise times of less than 10 microseconds, the two-counter circuit described will not perform adequately.
There exists a need for a method and apparatus for detecting the peak of an electrical pulse generated by a borehole scanning system.
There exists a need for such a method and apparatus in which each detected peak value is associated with a value representing the time between the occurrence of the detected peak and the preceding firing pulse.
There exists a need for such a method and apparatus in which the peak value of a rapidly rising electrical pulse can be accurately and inexpensively associated with a value representing the time between the occurrence of the detected peak and the preceding firing pulse.
There exists a need for such a method and apparatus in which the elapsed time between a firing pulse and the following electrical pulse is measured to approximately half way between the beginning and ending of the top of the electrical pulse.